Vulcanized rubbers, such as vulcanized styrene-butadiene rubber (SBR), butadiene rubber (BR) and natural rubber (NR), are popular soling materials because they possess a well balance of desirable properties such as abrasion resistance, tensile strength, tear strength and flexibility. However, the vulcanized rubbers can be difficult to process, and may lose the desirable properties when foamed.
Ethylene vinyl acetate copolymers (EVA), polyolefins (PO) and blends thereof are used to fabricate foam products in footwear applications such as inner soles, midsoles, outer soles, unisoles, and sole inserts. Formulating these foams often requires a tradeoff among desirable properties such as compression set resistance, rebound resilience, shrinkage, hardness, abrasion, flexibility, grip, and split tear properties. For example, a low foam density can be desirable in some applications where flexibility and good grip properties are needed. However, a low foam density generally results in low compression set, low abrasion resistance, and high foam shrinkage.
Foams with a low compression set, i.e., high compression set resistance, can be desirable in some applications such as footwear articles (e.g., shoe, boot, sandal, and the like). However, achieving a low compression set generally requires increasing the foam density, the cross-link density and/or the polymer crystallinity of the foams. Increasing the cross-linking density of the foams may increase foam shrinkage, and increasing polymer crystallinity may decrease rebound resilience and produce a “harder” foam. Moreover, the cross-linking density of foams for footwear articles generally has a practical upper limit because most footwear foams should remain sufficiently thermoplastic for thermoforming into a finished shape.
Foams with high rebound resilience generally are desirable for using as footwear foams to maximize energy return to the wearer. Rebound resilience can be increased by decreasing the polymer crystallinity and by increasing cross-linking density. However, a combination of low polymer crystallinity and high cross-linking density may contribute to high foam shrinkage and low split tear resistance.
Because of the importance of compression set resistance and rebound resilience properties for most of the footwear applications, the foams for such applications are generally cured above the ideal level. However, as mentioned above, a high cure level (i.e., high cross-linking density) can negatively impact the split tear and shrinkage properties of the foams.
For the above reasons, there are still needs for improved foams that have a good balance of desirable foam properties, such as compression set resistance, rebound resilience, shrinkage, hardness, abrasion, flexibility, grip and/or split tear properties.